Lukkulaisvaaraite, Pd14Ag2Te9, a new mineral from Lukkulaisvaara intrusion, northern Russian Karelia, Russia

Abstract

AbstractLukkulaisvaaraite, Pd14Ag2Te9, is a new platinum-group mineral discovered in the Lukkulaisvaara intrusion, northern Russian Karelia, Russia. In polished section crystals are ~40 mm across, rimmed by tulameenite and accompanied to varying degrees by telargpalite and Bi-rich kotulskite. Lukkulaisvaaraite is brittle, has a metallic lustre and a grey streak. Values of VHN20 fall between 339 and 371 kg mm–2, with a mean value of 355 kg mm–2, corresponding to a Mohs hardness of ~4. In plane-polarized light, lukkulaisvaaraite is light grey with a brownish tinge, has strong bireflectance, light brownish-grey to greyish-brown pleochroism and distinct to strong anisotropy; it exhibits no internal reflections. Reflectance values of lukkulaisvaaraite in air (R1, R2, in %) are: 40.9, 48.3 at 470 nm, 47.6, 56.4 at 546 nm, 52.1, 61.0 at 589 nm and 57.5, 65.2 at 650 nm. Five electron microprobe analyses of natural lukkulaisvaaraite gave the average composition Pd 52.17, Ag 7.03 and Te 40.36, total 99.61 wt.%, corresponding to the empirical formula Pd14.05Ag1.88Te9.06 based on 25 atoms; the average of nine analyses on synthetic lukkulaisvaaraite is Pd 52.13, Ag 7.31 and Te 40.58, total 100.02 wt.%, corresponding to Pd13.99Ag1.93Te9.08. The mineral is tetragonal, space group I4/m, with a = 8.9599(6), c = 11.822(1) Å , V = 949.1(1) Å3 and Z = 2. The crystal structure was solved and refined from the powder X-ray diffraction (XRD) data of synthetic Pd14Ag2Te9. Lukkulaisvaaraite has a unique structure type and shows similarities to that of sopcheite (Ag4Pd3Te4) and palladseite (Pd17Se15). The strongest lines in the powder XRD pattern of synthetic lukkulaisvaaraite [d(Å),I,hkl] are: 2.8323(58)(130,310), 2.8088(92),(213), 2.5542(66)(312), 2.4312(41)(321,231), 2.1367(57)(411,141), 2.1015(52)(233,323), 2.0449(100)(314), 2.0031(63)(420,240), 1.9700(30)(006), 1.4049(30)(246,426), 1.3187(36)(543,453). The mineral is named for the type locality, the Lukkulaisvaara intrusion in Russian Karelia.